This invention relates to a method for enhanced recovery of oil from a subterranean oil-containing formation. In particular, it relates to improving the performance of a water flooding medium by employing a nonionic, hydrophobically modified water-soluble polymer as the thickener in the medium.
In the early stages of the recovery of oil from subterranean formations pressure from natural water or gas often provides the driving force necessary to displace the oil and drive it towards the production well. If these natural pressures are initially insufficient or when they have become insufficient as a result of oil production, alternative techniques to produce driving forces for the displacement of the subterranean oil towards the production well must be employed. Foremost among these enhanced recovery techniques is water flooding. With water flooding, a medium such as water or, more frequently, brine is injected into the oil containing formation through an injection well. The water flooding medium helps to maintain reservoir pressure and when it comes into contact with oil, to drive it towards the production well.
Even after water flooding, however, a significant amount of oil, as much as two-thirds of the oil initially present in the formation, remains. Major problems which limit the amount of oil which is recovered through water flooding include fingering and low sweep efficiency.
The mobility of a water flooding medium through a porous medium, such as the sandstone which forms many oil containing subterranean formations, is of primary significance in determining how the water flooding medium will behave when confronted with the problems of fingering and low sweep efficiency. Mobility is defined as the ratio of permeability (K) to viscosity, where permeability is given by the equation EQU K=(q L/A.DELTA.p)
and is reported in darcies, and where
q is flow rate in cc/sec,
L is length in cm,
A is cross sectional area in cm.sup.2,
.DELTA.p is change in pressure over L in atmospheres.
It can be seen that mobility is the function of both the viscosity of a particular water flooding medium and the permeability of the system in which the water flooding medium is employed.
Where a fluid of low mobility, e.g. oil, is driven by one of a higher mobility, e.g. water, the higher mobility fluid tends to move through the low mobility fluid leaving the low mobility fluid behind. if this phenomenon, referred to as fingering, occurs while oil is being displaced by the water flooding medium, potentially recoverable oil will be left in the formation.
In order to overcome the tendency of the water flooding medium to finger through the oil, the water can be viscosified by the addition of a water-soluble polymer. As the viscosity of the water flooding medium is increased, its mobility is lowered and fingering is decreased. Consequently, oil recovery is enhanced.
Sweep efficiency is the ratio of the volume of rock contacted by the water flooding medium to the total volume of rock subject to invasion by the water flooding medium. Undesirably low sweep efficiency can arise where a water flooding medium having high mobility contacts subterranean areas of high permeability. When this occurs, the water flooding medium simply takes the path of least resistance and does not come into contact with the oil contained in less permeable areas. The sweep efficiency is improved by reducing the mobility of the water flooding medium. Where a water-soluble polymer is added to the water flooding medium, the mobility is decreased. This enables the water flooding medium to come into contact with areas of lower permeability so that it contacts more oil containing areas and consequently, oil recovery is improved.
The usefulness of polymers in controlling the mobility of water flooding medium can be ascertained by evaluating the water flooding medium in light of two related parameters. The first is resistance factor (hereinafter referred to as RF). RF is defined as the ratio of the mobility of a water flooding medium without polymer, to the mobility of the water flooding medium with polymer added. The second parameter is retained resistance factor (hereinafter referred to as RRF). RRF is the ratio of the mobility of the water flooding medium without polymer, measured before a water flooding medium with polymer added has been passed through a test core, to the mobility of the water flooding medium without polymer measured after a water flooding medium with polymer added has been passed through. Generally, high RF and low RRF values are indicative of polymers which are useful in water flooding media. However, polymers with a high RRF value have advantages in other enhanced oil recovery applications such as sealing off thief zones. Thief zones are areas in the subterranean formation which have high permeability and allow flooding chemicals to escape from the oil production zone.
Another problem limiting the recovery possible with water flooding is that because the water flooding medium is immiscible with oil, it is ineffective in displacing oil which adheres to the rock in the formation. This problem has been dealt with by injecting a surfactant. When injected into the formation surfactants release the entrapped oil through miscible displacement. Unfortunately the use of surfactants is expensive.
Other problems which affect the efficiency of oil recovery involve the durability of the polymer in the polymer-water flooding medium: The polymer must be tolerant to the salts present in the brine used as the water flooding medium or in the formation; otherwise the polymer will precipitate out of solution. The polymer must not be degraded by the shear forces which arise as the polymer is injected and forced through the formation. The polymer must have reasonable thermal and biological stability because it may take upwards of two years from the time the polymer-water flooding medium is injected until it makes its way to the production well. Furthermore, for polymer-water flooding to be economical, the polymer must be effective at low concentrations.
Most known water-soluble polymers have been evaluated in light of these problems. Poly(acrylamide), xanthan gum and hydroxyethyl cellulose are the currently preferred polymers. However, poly(acrylamide) is quite sensitive to shear forces and also to salts, particularly divalent metal salts. Xanthan gum is very expensive and can cause injectibility problems unless it is highly purified. Hydroxyethyl cellulose while the least expensive of these three is also the least efficient thickener. Moreover, none of these polymers exhibits the surface activity necessary to give rise to miscible displacement.
These problems are successfully countered by the method of this invention wherein a water flooding medium containing a hydrophobically modified, water-soluble polymer is injected into the oil-containing formation.